1. Field of the Invention
The invention relates to the field of photodetectors with internal gain.
2. Description of the Related Art
A photodetector with internal gain is a device which detects electromagnetic radiation and generates an electrical signal, said signal being amplified before it is processed by proximity electronics used to convert, amplify and/or digitize the signal produced by the photodetector.
The internal amplification of the photodetector makes it possible to reduce the amplification of the proximity electronics which is usually a source of noise. In addition, a photodetector with internal gain is currently the only device which is capable of quickly detecting a small number of photons; it is also the only device which allows accurate photon counting.
There are several types of photodetectors with internal gain, in particular photocathode detectors, electron-multiplier charge-coupled devices, avalanche photodiodes and solid-state impact ionization multipliers.
In a photocathode detector, incident radiation generates electrons on a photocathode. The released electrons are then accelerated through an amplifying element. Although they have a good signal-to-noise ratio, photocathode detectors are usually bulky, are limited in terms of their detectable wavelength range (usually less than 1.7 micrometers) and have a low quantum efficiency (generally less than 50%).
Electron-multiplier charge-coupled devices (EMCCDs) detect light in a first CCD array and the charges thus generated are transferred, pixel to pixel, to a second CCD array by applying repetitive voltage pulses. Each charge integrated in a pixel of the first CCD array is thus transferred N times and, by applying a sufficiently high voltage to said array, the transferred charge carriers can be subjected to a so-called “impact ionization” interaction which creates an additional electron-hole pair that produces an amplification gain m. The total amplification gain therefore equals N×m. The quantum efficiency of EMCCD devices is generally high for wavelengths in the visible spectrum but deteriorates rapidly in the near infrared spectrum. In addition, the spectral sensitivity of this type of device is limited by the material which is used for the CCD arrays, this is usually Si which has an energy gap which corresponds to a cut-off wavelength of 0.9 micrometer.
In Avalanche PhotoDiodes (APDs), incident photons generate electron-hole pairs in a semiconductor which has a first type of conductivity. The minority photocarriers are then collected by a highly reverse biased p-n junction. The strong electric field in the p-n junction then generates an impact ionization avalanche which amplifies the signal. APDs are rugged, compact detectors which are relatively insensitive to magnetic fields and have a high quantum efficiency, typically of the order of 90%.
APDs made of Si or of a Group III-V type semiconductor such as diodes made using InGaAs, for example, operate at ambient temperature, but their performance is limited by the presence of impact ionization which is produced by both the electrons and holes. At high gain, there is thus considerable deterioration of the signal-to-noise ratio as well as considerable deterioration of the response time. Avalanche breakdown can also occur; this makes APDs of this type non-linear.
CdxHg1-xTe-based APDs have a small energy gap which corresponds to cut-off wavelengths of 2.2 to 9 micrometers; they have properties which are close to ideal, thanks to impact ionization being initiated exclusively by a single type of photocarrier, namely electrons. APDs of this type exhibit exponential gain up to very high gains in excess of 5000, even with low bias voltages of tens of volts, as well as a very good signal-to-noise ratio. The excess noise factor (this is the ratio of the increase in the quantum noise of the photodiode in the avalanche regime to the quantum noise of the same photodiode in the non-avalanche regime) is close to 1.
However, CdxHg1-xTe-based APDs which have a small energy gap must be cooled in order to keep the dark current sufficiently low not to adversely affect their sensitivity.
For larger energy gaps, the performance of CdxHg1-xTe-based APDs deteriorates and becomes more similar to that of conventional APDs made of Si or a III-V type semiconductor.
What is more, the performance of CdxHg1-xTe-based APDs which have a small energy gap (less than 0.15 eV) deteriorates significantly due to the high dark current produced by the tunneling effect created by the electric field in the depletion zone.